In the mixed acid nitration of aromatic compounds, such as toluene, a mixture of concentrated nitric acid and concentrated sulfuric acid is contacted with an aromatic compound under reaction conditions such that nitroaromatic products including mono and dinitrobenzene and mono and dinitrotoluene are produced. In the mixed acid nitration, water and/or dilute sulfuric acid are generated as by-products. The bulk aqueous phase of spent acid is readily separated from the bulk organic phase comprising nitroaromatic and organic by-products by decantation. Prior to reuse, residual contamination of the sulfuric acid by nitric acid, nitrous acid, unreacted aromatic and organic reaction by-products is removed and the dilute sulfuric acid, as low as 65% by weight, is concentrated to as high as 95% by weight.
In one commercial process for purification and concentration of spent sulfuric acid, the sulfuric acid is recovered by decantation from the organic phase, heated and introduced to the top of a packed column for effecting denitrification. Steam is introduced to a bottom portion of the sulfuric acid denitrification column and nitric acid and derivatives are stripped therefrom. The denitrified sulfuric acid is removed from the bottom portion of the sulfuric acid denitrification column and heat exchanged against incoming feed to the column prior to storage. Sulfuric acid after denitrification then is heated and concentrated in a series of multiple effect evaporators.
Representative patents which describe various aspects of purification and concentration of sulfuric acid obtained as a spent acid from the mixed acid nitration of aromatic compounds are as follows:
U.S. Pat. No. 3,856,673 discloses a process for purifying spent acid obtained from the nitration of aromatic hydrocarbons with particular emphasis towards removing organic impurities contained therein. The patent describes in its prior art section a conventional method for removing impurities which comprised heating the spent sulfuric acid to its boiling point and stripping the acid with hot gases such as steam, air or nitrogen. The improved purification process set forth in the patent resided in admixing the sulfuric acid stream with an oxidizing agent selected from the group consisting of ozone, hydrogen peroxide, peroxydisulfates, and chlorates.
U.S. Pat. No. 4,157,381 discloses a process for the regeneration of dilute sulfuric acid containing organic impurities, and possibly inorganic salts, in a three-step process. In the first step, dilute sulfuric acid is heated in indirect heat exchange with vapor from a second step and directly heat exchanged with steam in the second step. Acid as it leaves the first step is contacted with a countercurrent flow of a mixture of steam and acid vapor and acid leaving the second step is contacted with superheated steam in direct heat exchange and the regenerated acid separated with the remaining mixture of steam and acid recycled to the second step. An oxidant is added in at least one of the steps to oxidize organic components.
U.S. Pat. No. 4,409,064 discloses a process for the concentration of sulfuric acid obtained as a spent acid from the mixed acid nitration of aromatic compounds. The patent describes equipment modifications in the form of mist elimination pads in a series of multiple effect evaporators to reduce carry over of entrained droplets in the overhead from these evaporators.
U.S. Pat. No. 4,155,989 discloses a process for the denitrification of spent sulfuric acid obtained as a by-product from various nitration processes. Sulfur dioxide is added to the spent nitration acid in sufficient amount to convert to nitric acid contained therein to volatile nitrogen oxides, vaporized nitrogen oxides are removed.
U.S. Pat. No. 3,972,987 discloses a process for the regeneration of sulfuric acid containing organic substances and mineral salts. The process comprises distilling the impure water-containing sulfuric acid in vacuo, drawing off a bottom product from the distillation and concentrating the condensate in a Pauling installation. In the Pauling process, sulfuric acid is passed through a dephlegmator mounted on a vessel. Hot vapors rising from the boiling contents of the vessel are stripped by cooled dilute acid flowing downwardly. To aid in decomposition of inorganic impurities, an oxidant is added to the process.